With the recent development of information and communication technology, a ubiquitous society based on information and communication technology has arisen.
In order to enable access to and by information sharing appliances without regard to time or place, sensors, which incorporate computer chips having a communication function therein, need to be installed in all public facilities. Thus, problems related to the supply of power to these appliances or sensors have newly arisen. In addition, as the kinds of portable appliances, such as, for example, mobile phones, Bluetooth handsets, and music players such as iPod, have rapidly increased, the task of charging a battery demands time and effort on the part of the user. As a method to solve this problem, a wireless power transfer technology has recently received attention.
A wireless power transmission (or wireless energy transfer) technology is a technology that wirelessly transfers electricity from a transmitter to a receiver using the principle of induction of a magnetic field. An electric motor or a transformer using the principle of electromagnetic induction has been used since the 1800's, and since that time methods of transferring electricity by emitting electromagnetic waves such as laser or radio waves have been attempted. Electric toothbrushes or some wireless razors that are often used are actually charged based on the principle of electromagnetic induction.
Wireless energy transfer methods that have been achieved thus far may be broadly divided into a magnetic induction method, an electromagnetic resonance method, and an RF transmission method using a short-wavelength radio frequency.
The magnetic induction method is a technology using a phenomenon whereby, when two coils are arranged close to each other and current is applied to one coil, a magnetic flux is generated to generate electromotive force in the other coil, and the commercialization of magnetic induction is quickly progressing in the field of small appliances such as mobile phones. The magnetic induction method may transmit power of a maximum of several hundred kilowatts (kW) and may have high efficiency. However, since the maximum transfer distance is cm or less, an appliance needs to be generally located close to a charger or a substrate.
The electromagnetic resonance method has the feature of using an electric field or a magnetic field, rather than using electromagnetic waves, current or the like. The electromagnetic resonance method is hardly influenced by an electromagnetic wave, and therefore is harmless to other electronic appliances or humans. In contrast, the electromagnetic resonance method may be used at a limited distance and in a limited space, and the energy transfer efficiency thereof is somewhat low.
The short-wavelength wireless power transfer method,—referred to in brief as an RF transmission method,—uses a method of directly transmitting and receiving energy in the form of radio waves. This technology is an RF type wireless power transfer method using a rectenna. “Rectenna” is a portmanteau of “antenna” and “rectifier”, and means an element that directly converts RF power into direct current (DC) power. That is, the RF transmission method is a technology of converting alternating current (AC) radio waves into DC radio waves and using DC radio waves. Recently, research into the commercialization of RF transmission has been actively conducted as the efficiency thereof has improved.
Such wireless power transfer technology may be variously used in all industries, such as, for example, IT, rail, and consumer electronics, in addition to the mobile industry.
Recently, in order to increase the rate of recognition of a wireless power receiver put on a charger bed, a wireless power transmitter in which a plurality of coils is mounted has been launched. The coils are formed in multiple layers, and insulation layers are required for electrical disconnection between the multiple layers. At this time, because each of the coils is formed in a Printed Circuit Board (PCB), which is patterned in a spiral structure, the thickness of each of the coils is limited to the thickness of the PCB or less.
The resistance of the coil is increased as the thickness of the coil is reduced, thereby having an effect on the power transfer efficiency of the wireless power transmitter. When the coils mounted in the wireless power transmitter are formed in the PCB, power transfer efficiency may be deteriorated because the thickness of the coils is limited to a predetermined thickness or less. In addition, the price of the PCB is relatively high, which may increase the overall production price of the wireless power transmitter.